The present invention generally relates to multicomponent ion exchange resins comprising granules comprising at least one crosslinked acidic resin and at least one crosslinked basic resin. Each superabsorbent ion exchange granule has at least one microdomain of the acidic resin in contact with or in close proximity to, at least one microdomain of the basic resin. The ion exchange resins can be used, for example, in the purification of water, sugar refining, recovery of transition metals, recovery of proteins from fermentation broths and agricultural by-products, and in pharmaceutical separations technology.
Ion exchange generally is defined as a reversible chemical interaction between a solid and a fluid, wherein selected ions are interchanged between the solid and fluid. An exemplary ion exchange process includes a exchange process wherein a fluid passes through a bed of porous resin beads having charged mobile cations or anions, such as hydrogen or hydroxide ions, which are available for exchange with metal ions or anions present in the fluid. The ion exchange resin readily exchanges hydrogen ions for the metal ions, or hydroxide ions for other anions, present in the fluid as the fluid passes through the bed.
In time, the number of hydrogen or hydroxide ions available for exchange with metal ions or other anions diminishes. Eventually, the resin becomes exhausted and cannot perform any further ion exchange (i.e., all available exchange sites are occupied). However, the resin can be regenerated. Regeneration is accomplished using a regenerant solution, which, in the case of a cation exchange resin, comprises an acid, i.e., a large excess of hydrogen ions, that is passed over the ion exchange beads and drives the collected ions from the resin, thereby converting the ion exchange resin back to its original form.
A specific example of a cation exchange process is the purification/softening of tap water. In this process, weak acid ion exchange resins use carboxyl radicals, in the sodium form, as the cation exchange site. The sodium ions are the charged mobile cations. Alkaline earth metals, such as calcium and magnesium, present in the tap water are exchanged for the sodium cations of the resin as the water passes through a bed of the ion exchange resin beads. Removal of calcium and magnesium ions from water in exchange for sodium ions via weak acid cation exchange resins is not limited to the water purification/softening applications, but also includes the softening of fluids, such as clay suspensions, sugar syrups, and blood, thereby rendering the fluids more amenable to further processing. When the exchange capabilities of the ion exchange resin are exhausted, a weak acid can be used to regenerate the acid form of the resin, followed by conversion of the acid form of the resin to the sodium form with dilute sodium hydroxide.
Similarly, an anion exchange resin containing anionic radicals removes anions, like nitrate and sulfate, from solution. Anion exchange resins also can be regenerated with a sodium hydroxide solution, for example.
The reversibility of the ion exchange process permits repeated and extended use of an ion exchange resin before replacement of the resin is necessary. The useful life of an ion exchange resin is related to several factors including, but not limited to, the amount of swelling and shrinkage experienced during the ion exchange and regeneration processes, and the amount of oxidizers present in a fluid passed through the resin bed.
Cation exchange resins typically are highly crosslinked polymers containing carboxylic, phenolic, phosphonic, and/or sulfonic groups, and roughly an equivalent amount of mobile exchangeable cations. Anion exchange resins similarly are highly crosslinked polymers containing amino groups, and roughly an equivalent amount of mobile exchangeable anions. Suitable exchange resins, preferably, (a) possess a sufficient degree of crosslinking to render the resin insoluble and low swelling; (b) possess sufficient hydrophilicity to permit diffusion of ions throughout its structure; (c) contain sufficient accessible mobile cation or anion exchange groups; (d) are chemically stable and resist degradation during normal use; and (e) are denser than water when swollen.
Hatch U.S. Pat. No. 3,957,698 discloses production of weak acid ion exchange resins by the suspension copolymerization of methacrylic or acrylic acid, in a low molecular weight hydrocarbon diluent, with 0.5 to 10 wt. % of divinylbenzene, based on the weight of initial monomers, to achieve the proper degree of crosslinking. In order to prepare high purity ion exchange resins by this process, the resin is heated at a high temperature or is extensively washed with solvents to remove the diluent. The resin particles were in the size range of 2 to 5 microns.
Meitzner et al. U.S. Pat. No. 4,224,415 discloses the preparation of ion exchange particles prepared by suspension copolymerization of water-insoluble monomers, such as methyl acrylate and methyl methacrylate, with a crosslinking agent, such as divinylbenzene. In addition, a precipitant is added to the monomer phase to impart a reticular nature to the resulting particles. This process requires a divinylbenzene content in the range from 8 to 25 wt. % to prepare the desired materials. The particles must be hydrolyzed with a strong base in order to prepare a material with exchangeable functionalities.
Therefore, conventional weak acid cation exchange resins typically are produced using a multistep process. The first step is a batch, aqueous suspension polymerization of methyl acrylate monomer, in the presence of divinylbenzene, to provide crosslinked beads of methyl acrylate. The poly(methyl acrylate) beads then are reacted with sodium hydroxide to hydrolyze the ester groups of the poly(methyl acrylate), and thereby introduce carboxylate (i.e., weak acid) functionality into the beads. Due to solubility of the acrylic acid in the aqueous phase of the suspension polymerization, acrylic acid is not wholly substituted for the methyl acrylate monomer in the foregoing process. Therefore, conventional manufacturing processes preferably utilize methyl acrylate, which is a relatively expensive monomer, in the syntheses of the ion exchange resin. Acrylic acid, however, can be copolymerized in a batch process with methyl acrylate monomer utilizing a divinylbenzene cross-linker.
The essentially total removal of ions from electrolyte-containing solutions is often accomplished using two ion exchange resins. In this process, deionization is performed by contacting an electrolyte-containing solution with two different types of ion exchange resins, i.e., an anion exchange resin and a cation exchange resin. The most common deionization procedure uses an acid resin (i.e., cation exchange) and a base resin (i.e., anion exchange). The two-step reaction for deionization is illustrated with respect to the desalinization of water as follows:
NaCl+Rxe2x80x94SO3Hxe2x86x92Rxe2x80x94SO3Na+HCl
HCl+Rxe2x80x94N(CH3)3OHxe2x86x92Rxe2x80x94N(CH3)3Cl+H2O.
The acid resin (Rxe2x80x94SO3H) removes the sodium ion; and the base resin (Rxe2x80x94N(CH3)3OH) removes the chloride ions. This ion exchange reaction, therefore, produces water as sodium chloride is adsorbed onto the resins. The resins used in ion exchange do not absorb significant amounts of water.
The most efficient ion exchange occurs when strong acid and strong base resins are employed. However, weak acid and weak base resins also can be used to deionize saline solutions. The efficiency of various combinations of acid and base exchange resins are as follows:
Strong acidxe2x80x94strong base (most efficient)
Weak acidxe2x80x94strong base
Strong acidxe2x80x94weak base
Weak acidxe2x80x94weak base (least efficient).
The weak acid/weak base resin combination requires that a xe2x80x9cmixed bedxe2x80x9d configuration be used to obtain deionization. The strong acid/strong base resin combination does not necessarily require a mixed bed configuration to deionize water. Deionization also can be achieved by sequentially passing the electrolyte-containing solution through a strong acid resin and strong base resin.
A xe2x80x9cmixed bedxe2x80x9d configuration of the prior art is simply a physical mixture of an acid ion exchange resin and a base ion exchange resin in an ion exchange column, as disclosed in Battaerd U.S. Pat. No. 3,716,481. Other patents directed to ion exchange resins having one ion exchange resin imbedded in a second ion exchange resin are Hatch U.S. Pat. No. 3,957,698, Wade et al. U.S. Pat. No. 4,139,499, Eppinger et al. U.S. Pat. No. 4,229,545, and Pilkington U.S. Pat. No. 4,378,439. Composite ion exchange resins also are disclosed in Hatch U.S. Pat. Nos. 3,041,092 and 3,332,890, and Weiss U.S. Pat. No. 3,645,922.
The above patents are directed to resins that can be used to remove ions from aqueous fluids, and thereby provide purified water. Ion exchange resins used for water purification must not absorb large amounts of water because resin swelling resulting from absorption can lead to bursting of the ion exchange containment column.
Ion exchange resins having a composite particle containing acid and base ion exchange particles embedded together in a matrix resin, or having acid and base ion exchange particles adjacent to one another in a particle that is free of a matrix resin are disclosed in B. A. Bolto et al., J. Polymer Sci.:Symposium No. 55, John Wiley and Sons, Inc. (1976), pages 87-94.
In view of the foregoing, it would be desirable to provide a multicomponent ion exchange resin produced from less expensive monomers such as, for example, acrylic acid. Furthermore, it would be desirable to provide a multicomponent ion exchange resin that can be manufactured in a simple, continuous process, and that exhibits the excellent ion exchange properties and physical properties of prior ion exchange resins, or improves upon these properties. Additionally, it would be desirable to minimize the amount of organic solvents used in the preparation of an ion exchange resin.
The present invention is directed to multicomponent ion exchange resins comprising at least one crosslinked acidic resin, such as a polyacrylic acid, and at least one crosslinked basic resin, such as poly(vinylamine), a polyethyleneimine, or a poly(dialkylaminoalkyl acrylamide) or a poly(dialkylaminoalkyl methacrylamide), hereafter collectively referred to as poly(dialkylaminoalkyl (meth)acrylamides), and to methods of manufacturing the same. More particularly, the present invention is directed to multicomponent ion exchange resin granules containing at least one discrete microdomain of at least one acidic resin in contact with, or in close proximity to, at least one microdomain of at least one basic resin. The multicomponent ion exchange granules can contain a plurality of microdomains of the acidic resin and/or the basic resin dispersed throughout the particle. The acidic resin can be a strong or a weak acidic resin. Similarly, the basic resin can be a strong or a weak basic resin.
A preferred ion exchange granule contains one or more microdomains of at least one weak acidic resin and one or more microdomains of at least one weak basic resin. The properties demonstrated by such preferred multicomponent ion exchange granules are unexpected because, in ion exchange applications, the combination of a weak acid and a weak base is the least effective of any combination of a strong or weak acid ion exchange resin with a strong or weak basic ion exchange resin. The present ion exchange granules contain discrete microdomains of acidic and basic resin, and during hydration, the granules are nonswelling.
One aspect of the present invention, therefore, is to provide a multicomponent ion exchange resin comprising a dry granular product that preferably is internally crosslinked with a bulk and a latent crosslinking agent, and optionally is surface crosslinked.
The bulk crosslinking agent, which provides internal crosslinks between the acidic resin polymer chains or the basic resin polymer chains, has at least two polymerizable carbon-carbon double bonds. The latent crosslinking agent, which also provides internal crosslinks through the pendant acid or amino groups of a polymer includes (a) compounds having at least one polymerizable double bond and at least one functional group reactive with an acid or an amino group, (b) compounds having at least two functional groups reactive with acid or amino groups, (c) polyvalent metal compounds capable of forming ionic crosslinks with acid groups, and (d) mixtures thereof. The surface crosslinking agent typically is a diglycidyl ether, a polyhydroxy compound, a hydroxyalkylamide, an alkylene carbonate, or a mixture thereof.
Another aspect of the present invention is to provide a method, preferably a continuous method, of manufacturing a multicomponent ion exchange resin. The present multicomponent ion exchange granules are produced by any method that positions a microdomain of an acidic resin in contact with, or in close proximity to, a microdomain of a basic resin to provide a discrete particle. In one embodiment, the multicomponent ion exchange granules are produced by coextruding an acidic hydrogel and a basic hydrogel to provide granules having a plurality of discrete microdomains of an acidic resin and a basic resin dispersed throughout the particle. Such ion exchange granules absorb less than about 25 times, and preferably less than about 15 times, their weight in tap water under no load (i.e., AUNL), and resist swelling to a sufficient degree to perform as an ion exchange resin.
In another embodiment, the present multicomponent ion exchange granules can be prepared by admixing dry particles of a basic resin with a hydrogel of an acidic resin, then extruding the resulting mixture to form multicomponent ion exchange granules having microdomains of a basic resin dispersed throughout a continuous phase of an acidic resin. Alternatively, dry acidic resin particles can be admixed with a basic resin hydrogel, followed by extruding the resulting mixture to form multicomponent ion exchange granules having microdomains of an acidic resin dispersed in a continuous phase of a basic resin.
In addition, a multicomponent ion exchange granules containing microdomains of an acidic resin and a basic resin dispersed in a continuous phase of a matrix resin can be prepared by adding dry particles of the acidic resin and dry particles of the basic resin to a hydrogel of the matrix hydrogel, then extruding. Other forms of the present multicomponent ion exchange granules, such as agglomerated particles, interpenetrating polymer network forms, laminar forms, and concentric sphere forms, also demonstrate good ion exchange properties.
In accordance with yet another important aspect of the present invention, the acidic and basic resins are internally crosslinked, such as with a suitable bulk crosslinking agent and latent crosslinking agent. In preferred embodiments, the acidic resin, the basic resin, and/or the entire multicomponent ion exchange granules are surface treated or annealed to further improve ion exchange properties.
Yet another aspect of the present invention is to provide a method of manufacturing an acidic resin or a basic resin by polymerizing one or more monounsaturated acid or base monomer, or a salt thereof, and optional vinyl monomers, in the presence of about 0.01 to about 3 mole %, based on the total number of moles of the monomers, of a polyvinyl bulk crosslinking agent, to provide a bulk crosslinked polymer hydrogel, followed by the addition of about 0 to about 6 mole %, based on the total number of moles of the monomers, of a latent crosslinker to further crosslink the bulk crosslinked hydrogel, then heating the hydrogel-latent crosslinking agent mixture at a sufficient temperature for a sufficient time to dry and cure the hydrogel and to form crosslinks through pendant acid groups present on the polymer chains.
The multicomponent ion exchange granules can be surface crosslinked with 0 to about 2 weight % of a surface crosslinking agent, based on the weight of the granules. The step of surface crosslinking can be achieved by coating surfaces of the ion exchange granules with a solution containing a surface crosslinking agent, and then heating the coated granules at a sufficient temperature and for a sufficient time to dry the granules and provide surface crosslinks. A surface crosslinking agent often is required to provide the degree of crosslinking needed for an ion exchange resin.
A present multicomponent ion exchange resin preferably is in the form of irregular granules, as opposed to spheres. The granular form of the ion exchange resin provides a greater surface area than spheres, thereby providing a more efficient ion exchange. In some embodiments, the present ion exchange resin granules generally have a relatively xe2x80x9csoftxe2x80x9d interior, i.e., the degree of internal crosslinking (i.e., bulk plus latent crosslinking) is sufficient to resist granule swelling and to allow percolation of a liquid through the particles. The granules, however, typically have a relatively xe2x80x9chardxe2x80x9d surface, i.e., the degree of surface of surface crosslinking is high, i.e., at least 1000 ppm, and preferably about 1000 to about 20,000 ppm, to impart structural integrity to the granules and to further prevent swelling of the granules during use.
An example of a weak acid resin is polyacrylic acid having 0% to 25% neutralized carboxylic acid groups (i.e., DN=0 to DN=25). Examples of weak basic resins are a poly(vinylamine), a polyethylenimine, and a poly(dialkylaminoalkyl (meth)acrylamide) prepared from a monomer either having the general structure formula (I) 
or the ester analog of (I) having the general structure formula (II) 
wherein R1 and R2, independently, are selected from the group consisting of hydrogen and methyl, Y is a divalent straight chain or branched organic radical having 1 to 8 carbon atoms, and R3 and R4, independently, are alkyl radicals having 1 to 4 carbon atoms. Examples of a strong basic water-absorbing resin are poly(vinylguanidine) and poly(allylguanidine).
Further aspects and advantages of the invention will become apparent from the following detailed description of the preferred embodiments of the invention, taken in conjunction with the examples and the appended claims.